The present invention concerns an oxidation and biological reduction reactor, a filtration process and washing procedures used in this reactor.
Such an oxidation and reduction reactor especially applies to the elimination of the organic and ammonia nitrogen found in urban and industrial waste waters. The oxidation transforms the ammonia nitrogen into nitrite and then into nitrate. The reduction transforms the nitric nitrogen into nitrogen These reactions are obtained by using bacterial cultures and may be carried out according to two methods, one called the free culture method, the other called the fixed culture method.
In the fixed culture of the prior art, the ability of most micro-organisms to produce exopolymers enabling their fixation on very different backings is used to form a biofilm. The fixed cultures as well as the free cultures may be used with aerobic or anaerobic treatments (fine granular medium biofilters, biofilters, biodisks, etc.).
Fixed cultures are used to obtain higher biomass concentrations (and sometimes activities) in the reactors, allowing the size of the works to be reduced.
A recommendation has been made to fix the microorganisms on granular backings whose effective bead size does not exceed 4 or 5 mm. These backings provide a specific developed surface (and thereby an exchange surface) that is much greater than that of other methods. By way of example, BIOLITE with an effective bead size of 2.7 mm develops an exchange surface of 700 m.sup.2 .m.sup.-3.
Roughly, these granular medium bioreactors may be divided into two main categories:
Those where the matter in suspension initially present in the raw wastes as well as the excess sludge produced is retained at the same time as the biological purification. The fixed granular filters are also called biofilters.
Those in which the bioreactor only provides biological purification and the matter in suspension is separated by works found downstream. In the latter case, in order to optimize the micro-organism/substrate exchange surfaces, very fine granular materials in movement are used. These are the fluidized granular beds, as indicated in U.S. Pat. Nos. 3 846 289 and 4 009 099 (Ecolotrol denitrification and nitrification).
In the case of the second category, the fluidized bed reactors have the disadvantage of relating the speed of circulation of the fluid to the hydraulic characteristics (speed of fluidization, etc.) of the granular material chosen as a backing. Moreover, the fluidized beds do not enable the reliable and permanent maintenance of two separate zones, one of oxidation, the other of reduction in the same bed without the risk of mixing the materials and bacteria retained on these materials during variations in discharge.
In the first category, the "fixed granular beds" described above are classically made with materials that are more dense than water. In this case, we can speak of a "flowing bed". We can speak of a "floating bed" when the material is not as dense as water.
Patent FR 2 604 990 (OTV) describes a device consisting of a flowing bed comprising at least two layers of granular materials. Two zones, one aerobic and the other anaerobic are found within this bed. There are several disadvantages to this process:
the granulometries of the materials of both zones should be chosen so as to avoid the mixture of the grains in the adjacent zones;
the washing requires very fast water although the reclassification of the different layers is not foreseen in case there is a mixture of the zones.
Devices using floating beds have been recommended:
For example, patent FR 2 330 652 (Preussag) proposes the use of floating materials such as expanded polystyrene to create a biological denitrification reactor. However, this process can not be used to carry out the biological nitrification reaction since it only comprises one zone adapted to one reduction reaction (denitrification). This arrangement requires a large chamber at the upper part for the accumulation of washings in order to enable washing with a downward flow.
Several patents describe the devices using two layers of different materials in the reactor:
Patent FR 2 278 378 (Erpac) recommends the successive crossing of one floating bed layer and then two flowing bed layers in order to carry out the filtration. However, this does not enable a biological reactor to be envisaged since there is no injection of air. It has the disadvantage that it includes several means of support and retention or compaction of the materials. In addition, the washing conditions for the different beds are not favourable.
Patent GB 2 021 428 (Ishigaki Kiko) comprises a floating bed layer and an upper flowing bed layer separated by an intermediate wall. As in the previous patent, it does not enable a biological reactor for an oxidation reaction and a reduction reaction to be considered since there is no air injection for the process part and there is no introduction of washing air. In fact, the lower layer is designed for the filtration and the upper layer for the physico-chemical adsorption is not designed for washing.
Patent FR 2 632 947 (OTV) uses a bed consisting of layers serving as a biological reactor. It considers using these layers for the nitrification or denitrification. However, if both nitrification and denitrification reactions are to be combined within the same reactor, the characteristics of the materials from both layers may not be chosen independently. The characteristics of these two layers are related by the constraints resulting from the washing conditions. In fact, the washing is carried out with expansion since it is necessary to maintain the classification of the different layers
Moreover, the oxidation gas circulates in the direction of the compression of the bed leading to a blockage of the circulation of the gas. This is the main problem that this patent wants to solve by choosing a density of material for the lower layer so that the layer is "agitated" (the patent indicates "fluidised"). The material, such as expanded floating slate or light polypropylene, which has such a density, is not available at a cost that is economically reasonable on an industrial scale.
This compression of the bed is therefore not favourable for the part of the bed forming the three-phase aerobic reactor In addition, this patent foresees the intermittent injection of the air (the patent indicates "by pulsation") to limit this disadvantage. In fact, there is a risk of air embolism and the preferential passage of the water or gas when the progressive fouling of the bed increases the loss of water circulation load
The disadvantage of the prior art process is that it does not allow effective and economic nitrification and denitrification reactions (or more generally oxidation and reduction) in the same reactor.